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What are galaxies?

In modern astronomy is the most widely used is the first classification of galaxies proposed by Edwin Powell Hubble in 1926, and subsequently modified, and then Gerard de Oculinum and Alan Candidum.

This classification is based on the form of known galaxies. According to her, all galaxies are divided into 5 main types:

- elliptical (E);

- spiral (S);

- a spiral galaxy with a jumper bar (SB);

- irregular (Irr);

- lenticular (S0) - this class Hubble added last, 10 years after the first of the proposed classification.

Galaxies too faint to be classified, Hubble identified by the symbol Q.

In addition, in the notation of galaxies in this classification are numbers indicating how plusnut elliptical galaxy, and letters to indicate how tightly the sleeve spiral galaxies are adjacent to the core.

Graphically this classification is presented as a number, which is called the Hubble sequence (or the Hubble tuning fork due to the similarity diagrams with this tool).

The Hubble Sequence

Elliptical galaxy (type E) make up 13% of the total number of galaxies. They look like a circle or ellipse, whose brightness decreases rapidly from the center to the periphery. Form elliptical galaxies are very diverse: they can be both a ball and a very flattened. In this regard, they are divided into 8 subclasses from E0 (ball shape, no compression) to E7 (most compression).

Elliptical galaxy M49 in the constellation Virgo (type E4)

Elliptical galaxies are the most simple. They consist mainly of old red and yellow giants, red, yellow, and white dwarfs. No dust matter. The formation of stars in galaxies of this type is not for several billion years. Cold gas and cosmic dust in them is almost there. The rotation is detected only in the most compressed of elliptical galaxies.

Spiral galaxies are the most numerous type: they are about 50% of all observed galaxies. The majority of stars in a spiral galaxy is located within the galactic disk. The galactic disk is visible spiral pattern of two or more twisted to one side branches or sleeves leaving the center of the galaxy.

Spiral galaxy M81 in URSA major (the Galaxy Bode, type d)

There are two types of spirals. The first type, denoted by SA or S, the spiral arms emerge directly from the Central seal. The second they start at either end of the oblong education, in the centre of which is an oval seal. The impression that the two spiral arms are connected by a jumper, making such galaxies are called crossed spirals; they are indicated by the symbol SB.

Spiral galaxy with a jumper NGC 1300 in the constellation of Eridan (type SBbc)

Spiral galaxies differ in the degree of development of its spiral structure, which in classification notes adding to symbols S (or SA) and SB letters a, b,C.

Sleeves spiral galaxies have a bluish color, as there is a lot of young giant stars. All spiral galaxies rotate with great speed, so the stars, dust and gases are concentrated in their narrow disk (stars of Population I"). Rotation in the vast majority of cases occurs in the direction of winding of the spiral branches.

Each spiral galaxy has a Central condensation. Color condensations of spiral galaxies - reddish-yellow, indicating that they are composed primarily of stars of spectral classes G, K, and M (i.e. the smallest and cold).

The abundance of gas and dust clouds and the presence of bright blue giants of spectral classes O and b says about the active processes of star formation occurring in the spiral arms of these galaxies.

In some galaxies the Central part has a spherical shape and brightly lit. This part is called bulge (from the English. bulge - thickening, swelling). Bulge is composed of old stars "Population II" and, often, a supermassive black hole in the center. The other galaxies in the Central part is the "star jumper" bar.

The most well-known spiral galaxy is our milky Way Galaxy and the Andromeda nebula.

Lenticular galaxy (type S0) is an intermediate type between spiral and elliptical galaxies. The galaxies of this type bright Central condensation (bulge) highly compressed and is similar to the lens, and the branches are absent or very poorly traced.

Are lenticular galaxies from old stars giants, so their color is reddish. Two-thirds of lenticular galaxies, like the elliptical, do not contain gas, one-third of the gas content is the same as spiral galaxies. Therefore, the process of star formation are very slow pace. Dust in lenticular galaxies are concentrated near the galactic core. To lenticular galaxies is about 10% of the known galaxies.

For incorrect or irregular galaxies (Ir) characteristic irregular, shaggy form. Irregular galaxies are characterized by the absence of a Central sealing and symmetrical patterns, as well as a low luminosity. These galaxies contain a lot of gas (mostly neutral hydrogen) up to 50% of their total weight. This is about 25% of all stellar systems.

Irregular galaxy NGC 1427A in the constellation of Eridan (IBm type)

Irregular galaxies are divided into 2 large groups. The first one, denoted by Irr I include galaxies with a hint of some structure. Division Irr I not final: thus, in studying the galaxy is detected similarity spiral arms (typical for galaxies of type S), the galaxy gets the designation Sm, or SBm (has its jumper); if such phenomena are not observed - symbol Im.

The second group irregular galaxies (Irr II) are all the other galaxies with chaotic structure.

There is a third group irregular galaxies, dwarf, denoted as dI or dIrrs. It is believed that dwarf irregular galaxies similar to the earliest galactic education that existed in the Universe. Some of them are small spiral galaxies, tidal forces destroyed more massive companions.

Typical representatives of such galaxies is Large and the Small Magellanic Cloud[?]. In the past it was believed that the Large and Small Magellanic clouds belong to irregular galaxies. However, it was later discovered that they have a helical structure with a bar. Therefore, these galaxies were reclassified in SBm, the fourth type spiral galaxies with a bar.

Galaxies that have those or other individual characteristics that do not allow to attribute them to any of the above classes are called peculiar.

An example of peculiar galaxies - radio galaxy Centaurus A (NGC 5128).

The radio galaxy Centaurus A (NGC 5128)
in visible light

The radio galaxy Centaurus A (NGC 5128).The image in the range rentgenovskim

Classification Hubble is currently the most common, but not the only one. In particular, widely used System de vocular representing more extended and revised version of the classification of Hubble, and Yerxa system in which galaxies are grouped according to their spectra, shape and degree of concentration to the center.